1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory device and, more particularly, to a non-volatile semiconductor memory device in which each memory cell includes a non-volatile memory cell and a capacitive element, and which operates as a high-speed DRAM (Dynamic Random Access Memory) in a normal operation mode and operates as a non-volatile memory in a data retaining mode.
2. Description of the Related Art
Generally, there are two kinds of semiconductor memory devices. One is a non-volatile memory (such as EEPROM) in which stored contents are retained even after a power supply is turned off. The other is a volatile memory (such as RAM) in which the stored contents disappear when the power supply is turned off.
The non-volatile memory having the above-described advantage has been remarkably progressed and has reached flash memories having a large capacity which are applied to various kinds of commercial products. Generally, a period of time required for rewriting in non-volatile memories is relatively longer than that of random access memories such as DRAM or SRAM. In order to obtain at least some improvement, the following measures have been taken. For example, in flash memories of NOR type, a rewriting speed in each memory cell is made higher by employing a CHE (Channel Hot Electron) method. In flash memories of NAND type, the rewriting speed is made higher by rewriting a large number of cells in parallel by using FN (Fowler Nordheim) tunnel current.
However, in conventional non-volatile memories such as a flash memory of NOR type or a flash memory of NAND type mentioned above, the shortest available period of time required for rewriting is about 1 .mu.s/ byte. This period of time is longer than the rewriting speed of DRAM or SRAM, which is about several ten nano second.
Thus, there has been an eager desire for the development of a non-volatile semiconductor memory device which can retain the above advantage as a non-volatile memory and which can allow a high-speed random access to such an extent as in an ordinary DRAM.